256 THE WORLD MACHINE 



the Principia ; as a matter of fact the problem, if not the proof, 

 was elementary. He had doubtless read Kepler's conjectures 

 that the attractive force of the sun varies in some fixed pro- 

 portion, simple, or as the square of the distance. Galileo had 

 shown that under the force of gravitation the speed of a falling 

 body increases with the square of the time. Perchance this 

 same force of gravity, which the fall of the apple may have 

 put in his mind, is the attractive force of the sun. It will be 

 likewise the retaining force which holds the moon as well. The 

 moon is about sixty times as distant from the centre of the 

 earth as are we upon the earth's surface. If the force of gravi- 

 tation projects outward to the moon, and it varies inversely as 

 the square of the distance, then the earth's pull upon the moon 

 will be just sisVu what it would be at the earth's surface. If, 

 under the influence of this force, bodies fall in proportion as the 

 square of the time, then the moon in a second will fall towards 

 the earth a distance proportional to the square root of 3600 in 

 other words, at sixty times the distance it will fall as far in a 

 minute as a body at the earth's surface does in a second. 



As the story goes, this lad of twenty-three tries it, taking 

 the then accepted figures as to the diameter of the earth and, 

 accordingly, the distance of the moon. It does not work out. 

 It ought to fall sixteen feet per minute ; the calculation says 

 less than fourteen. A man like Kepler, feeling the discovery so 

 near, would have worked at it night and day, as he did at his 

 Laws, for years and years. Newton was not metal of that temper. 

 He put his calculations aside. He does not doubt that the 

 law of duplicate proportion, as he calls it, is involved ; but it 

 may not be the whole explanation. Possibly Descartes' vortices, 

 with visions of which his head no doubt is then full, may play 

 some role ! Evidently for him Torricelli's experiment has yet 

 not swept clean the spaces of the sky. It is strange that he 

 thinks of the matter as of so little importance ; but in truth 

 his head is then full of questions in optics. As soon as he gets 

 a chance he will be buying prisms and lenses, inventing tele- 

 scopes and discovering the spectrum. Consider that all these 

 ideas come before he is twenty-five, and you will scarce wonder 

 if he hardly sees the drift of some of them himself. 



Five or six years after, Picard made a more accurate measure 

 of the size of the earth ; it appears that it was some years later 

 still when, in London, Newton hears a discussion in which the 



